Equipment packages are often placed on the ocean floor, at the surface or within pipelines by using on a cable. The cable can be of steel construction or synthetic materials with different advantages for each. Synthetic rope is the newer technology and offers features such as near neutral buoyancy in water and ease of terminations. A disadvantage of the synthetic ropes is that if wound layer over layer on a conventional drum like the steel cables, the outer layers will “knife” into the lower layers and tend to destroy the cable.
For this reason, the storage and low load pre-tensioning of the synthetic rope is handled by a first winch or reel and a second dual drum winch specific to handling synthetic rope handles the higher load. These special dual drum winches are typically dual drum traction winches. The traction name is because they use multiple wraps around multi-groove drums to accumulate a lot of friction or traction. Literally the friction from one drum groove is passed on to and is amplified by the next drum groove it crosses. Enough drum grooves are provided to amplify the initial tension up to a working level. They require an initial tension or pre-tension to work at all or the cable just slips on the drums.
As the dual drum traction winches require a pretention to be held on the synthetic rope coming into these special winches, it means two different types of capital equipment must be delivered to the job to utilize the synthetic rope. A pretention reel and a high capacity dual drum traction winch are both required. This reasonably eliminates bringing the synthetic rope to the jobsite on a wooden spool and simply putting it on a support stand or dispensing the synthetic rope from a wooden box or directly off the deck or floor.
Another complication is that the pretension provided by another reel is somewhat unpredictable. On the outer wraps at the larger diameters the motor or brake providing the back tension or pretension will give a certain value. At the inner wraps when the radius to the cable or rope is reduced, the back tension or pretension value goes inversely proportionate to the radius unless special care is taken. Adding to this the actual tension in the line between the pretension device and the dual drum traction winch is usually difficult to know. The load on the dual drum traction winch will be known as you are picking up a specific weight. Between the back tension or pretension device and the dual drum traction winch, you have no convenient measure like this. This means that in some cases a significant percentage of the dual drum traction winch capacity can be reduced by the amount of the hard to predict pretension loading.
This problem has persisted as long as cable or rope has been handled by the dual drum traction winches, with all the extra costs and complications which are inherent in providing duplicate pieces of capital equipment to do a single job.